For what qualities did Frederick II Staufen receive the nickname Stupor Mundi from his contemporaries?

For what qualities did Frederick II Staufen receive the nickname Stupor Mundi from his contemporaries? What did it mean? Why has none of the other emperors or popes deserved such a nickname?

Mundi’s stupor means “wonder of the world.” Frederick II earned this nickname because he knew not only many languages well, but also Islamic civilization, including its administrative culture (professional officials) and capitalist economy. Frederick II acquired such unique knowledge in childhood, when he lived in Sicily, in the former Arab possessions. All other emperors and popes of the 13th century were from Germany, France or Northern Italy, they did not understand the merits of the highest achievements of Islamic civilization.